If you don't know it, you can waste a lot !

When registering new templates in WEBTEMP.XML and WEBTEMPSPS.XML, you have to set an ID greater than 10000 (maybe Microsoft hopes to release a lot of new templates in the SP). And IDs must be different in the two files. For example, you can't have a template id of 10001 in the WSS definitions (WEBTEMP.XML) and another with id 10001 in the SPS areas definitions (WEBTEMPSPS.XML).

Have a good templating time ! :-)